Influenza viruses actually infect wild and domestic birds, swine (pigs),
and other animals, which makes them almost impossible to eradicate --
especially since most birds, including wild birds, can carry the viruses
without getting sick. Some of the antigens in swine, avian (bird), and
human influenza can trade places with each other and produce new and
different strains of viruses.
Swine flu, like human flu, is most common during the fall and winter.
However, the flu viruses are always present in some swine (as they are
in some people) in the off-season.

According to estimates from the
US Centers for Disease Control
and Prevention, between 34,000,000 and 67,000,000 cases
of humans infected with an H1N1 influenza A virus occurred in the US
between April 1 and November 14, 2009,
with cases confirmed in all 50 states, the District of Columbia, Puerto
Rico, and the US Virgin Islands. Between 154,000 and 303,000 patients
were admitted for problems related to the H1H1 strain and between
7,070 and 13,930 deaths have been attributed to this flu strain.
The CDC stopped reporting numbers of cases and deaths on a state-by-state
basis in late July and is now reporting numbers by US Department of Health
and Human Services regions. As of December 26, only 4 states (Delaware,
Maine, New Jersey, and Virginia) reported "widespread" flu activity,
down from 7 states the previous week. 13 states (Alabama, Alaska,
California, Connecticut, Florida, Georgia, Hawaii, Indiana, Kentucky,
Nevada, New Mexico, New York, and Tennessee) have reported "regional"
flu activity (flu cases limited only to certain areas of each state),
and 19 states, as well as the District of Columbia, have reported
flu activity limited to localised areas.
The map below shows the progression of swine flu cases in the United
States from the earliest cases at the end of April to early June, when
cases were reported in every US state for the first time (darker colours
correspond to higher numbers of confirmed cases).

As of December 26, 2009,
the Michigan Department of Community Health reported 692,986 cases
of flu-like illness in Michigan, compared with 416,972 cases in all of
2008 and 404,476 in all of 2007. There have been 2006 hospital admissions
and 64 deaths associated with influenza of any type since September 1,
2009 in Michigan.

Apparently the new strain of flu virus contains genetic material from
swine, bird, and human flu viruses found in North America, as well as
swine flu viruses found in Europe and Asia. Thus, this virus has been
popularly referred to as "swine flu", although there has been only one
report of pigs being infected with this strain (in Canada, in a pig
exposed to a farm worker who had recently been in Mexico. Remember
that influenza can be spread from one person or animal to another
without physical contact -- sneezing can be enough). Most people
who have died from complications of this flu strain in the US had other
medical problems which may have made them more susceptible to influenza.
This flu outbreak seems to be affecting mostly children and young adults,
while seasonal flu usually hits the elderly hardest, and most reported
cases have been mild (the CDC
suspects that most people with the "swine flu" are not being tested
because their symptoms are so mild, and that there may actually be over
100,000 cases in the US).

As of December 30, 2009, the
World
Health Organization (WHO) has reported at least 12220 deaths worldwide,
with cases in over 208 countries. Central and Eastern Europe have had
the most active recent flu activity, while activity has been declining
in other parts of the world including North America.
On June 11,
WHO
declared a phase 6 alert (sustained outbreaks in communities in
two regions of the world, which constitutes a global pandemic).
An explanation of the alert levels can be found at
the WHO Web page on pandemic alerts.

Vaccines are now available for this strain;
although the H1 and N1 antigens are similar to those in recent strains
of human flu, they are not similar
enough that the current
flu vaccine is expected to protect people from this swine flu strain.
A report from CDC
suggests that people in their 50's and 60's have some immunity to this
strain, and that the "new" strain is actually a descendant of the
1918 "Spanish" H1N1 flu virus, rather than the H1N1 strains of recent
years. The strain appears so far to be resistant to the
uncoating antivirals (amantidine and rimantadine), but sensitive to the
neuramidase
inhibitors (zanamivir and oseltamivir), although there have been recent
reports from several countries around the world of people infected with a
virus of this strain that is resistant to oseltamivir (but not
zanamivir).

People in the United States who have died of this strain have generally had
other medical problems, such as morbid obesity
(body mass
index of 35 or higher),
asthma,
diabetes, heart disease, or pregnancy, which made them more susceptible
to complications of the flu. My personal speculations on why (and
remember that I am not a microbiologist,
an infectious disease specialist, or an epidemiologist):

This flu strain started infecting people in Mexico and seems to have
been rather widely spread there before appearing in other countries.
By the time the outbreak was obviously under way many people in Mexico
were already infected while other countries have been watching for
infections partly because of the Mexico outbreak. The rising number
of confirmed cases, especially in the US, is also more likely related
to backlogs in confirmatory testing than to higher rates of infection
and the larger number of cases in the US is likely due to better
availability of the confirmatory tests. Also, there may be many
people around the world who have this flu strain but aren't sick
enough to be tested -- there are published estimates that hundreds
of thousands of people in the US have been infected, but most of
them are just shrugging off the infection.

It's possible that those areas where the most deaths have occurred are
those where medical care isn't as easily available as in other areas.
Delay in treatment of severe cases of flu make dying of the flu and
its complications more likely.
WHO
reported last week that indigenous peoples in many countries, including
the United States' Native American population, people of the Canadian
First Nations, and similar populations, are more likely to be severely
affected by this strain, and pointed out that these peoples' increased
risk of harm may be due in part to their not being able to find medical
care or to their higher chances of having chronic medical problems that
make then more vulnerable to influenza.

Frankly, this seems to be a rather wimpy little bug, compared with
some of the flu viruses and other infectious agents we've seen in the
past -- for example, the 1918 pandemic "Spanish" flu. However, it
is clearly capable of killing people, and has been killing people
around the world.

The best preventative measures are still good hygiene. The
CDC recommends:

using a tissue to cover your mouth and nose when you cough or sneeze,
and throwing away your tissue afterwards.

good handwashing with soap and water or with hand cleaners that contain
alcohol.

avoiding sick people when you can.

staying home from work if you have symptoms or signs of flu.

receiving vaccine against the H1N1 strain, in addition to
vaccine against this year's
seasonal
flu vaccines.

A surprising number of people are refusing to get the H1N1 vaccine,
for themselves or for their children. It's not clear why they
would want not to receive the vaccine -- especially for their children,
in view of the number of children who have died of H1N1 flu and its
complications.

One common "problem" that I have talked about with many parents is that
the H1N1 vaccine is "new". It isn't new. The H1N1 vaccine is
made in exactly the same way as the seasonal flu vaccine (by
growing the virus on chicken eggs); the only difference between
the H1N1 and seasonal flu vaccines is in the flu virus antigens
contained in each vaccine (the H1N1 vaccine contains only the H1N1
antigen, while the seasonal vaccine contains antigens to all three
of this year's
most likely flu strains.

Another issue is that some people remember the 1976 swine flu problem,
and especially the reactions many people had to the 1976 swine flu
vaccine. The vaccine people are much better at making vaccines
now than they were 33 years ago, and because of that the vaccines we
now have available are much safer, with fewer side effects, than they
were in the '70s. I haven't seen anyone have problems with the H1N1
vaccine. (We have seen a number of children die from H1N1
infections and its complications, though.) It still appears to be
much safer to receive the vaccine than to risk getting H1N1 flu.

For those people who eat pork products: it is impossible to
catch swine flu by eating well-cooked pork products. Pork and pork
products (such as ham and bacon) should always be cooked
thoroughly anyway, to eliminate any possibility of infection with
trichinosis, a parasite which is often found in pork products.

Also, there have been NO concerns about the safety of
vaccine from these lots -- the low antigen concentration is the only
problem with these lots of vaccine Vaccines from these lots are
being recalled, but children who have had vaccine from these lots
do not need to repeat the dose (although children younger than 10
years old who have had only one dose of H1N1 vaccine still need their
second doses).

PLEASE NOTE: As with all of this Web site, I try to give
general answers to common questions my patients and their parents ask me
in my (real) office. If you have specific questions about your
child you must ask your child's regular doctor. No doctor can give
completely accurate advice about a particular child without knowing and
examining that child. I will be happy to try and answer
general questions
about children's health, but unless your child is a regular patient of
mine I cannot give you specific advice.